This invention relates to a maximum-likelihood (ML) multiuser detector for direct-sequence code division multiple access (CDMA) and, more particularly, to a reduced-complexity multiuser detector based on the trellis structure of the ML detector.
There is an ever increasing demand or communication of different types of information, such as voice, data and video. In telecommunications systems, use may be made of frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). CDMA has a number of advantages over the other forms of communication, these advantages having led to the IS-95 standard for direct-sequence CDMA (DS-CDMA) for mobile communications, and a similar standard for personal communication systems (PCS). In uplink transmission (mobile users to base stations) of a system based on the IS-95 standard, the users transmit asynchronously with resulting interference between users.
In many CDMA applications, the performance of the communication system is limited by the interference from other users of the communication system. This situation is made worse if one or more of the interfering user signals reaches a receiver with higher power than the power received from a desired user. This results in what is commonly referred to as the near-far problem, and causes unacceptably high bit-error rates if nothing is done to reduce the resulting interference. Therefore, in an interference-limited system, it is important to reduce the multiple user interference (MUI) There are circumstances in which the MUI has a known structure which can be exploited to develop multiuser detectors that improve the performance of both synchronous and asynchronous CDMA channels. However, a problem arises in that existing solutions for reduction of multiple user interference do not produce a desired level of performance.
The aforementioned problem is overcome and other advantages are provided by the ML detector providing improved performance in the presence of MUI. In accordance with the invention, the ML detector can be implemented using the Viterbi algorithm, this greatly reducing system sensitivity to MUI. This is accomplished in accordance with the invention, by using an approximation to be ML detector which does a sparse trellis search based on the structure of the ML detector. The resulting detector, which may be referred to as a reduced-complexity recursive detector (RCRD), has a dynamic structure that allows a trade-off between complexity and performance. The trellis structure of the RCRD is similar to the trellis structure found in other communication subsystems such as trellis codes, equalizers for the removal of inter-symbol interference, and the modulation of digitally phase modulated signals. The RCRD may be integrated with a decoder for the error-control code (ECC) in a coded CDMA system.
The invention provides a flexible trellis-based method for improving the performance of a multiuser receiver for asynchronous CDMA. The procedure consists of using a unique metric to define the trellis-structure and a unique algorithm to reduce the number of surviving paths. The metric calculation is then repeated at decision points to provide soft-decision information for further signal processing, soft-decision decoding of an error-correction code, or iterative reception of the multiuser signal. The invention provides a variable-complexity model for providing reliable multiple point-to-point communications in an asynchronous CDMA system. The invention allows the user to trade complexity for performance and demodulate a large number of user communications.